Donald Dossey, author of "Holiday Folklore, Phobias and Fun: Mythical Origins, Scientific Treatments and Superstitious 'Cures,'" told National Geographic, "It's been estimated that $800 or $900 million is lost in business on this day because people will not fly or do business they normally would do."

What's the origin behind the superstition?

There are several theories:

Twelve is considered a complete number.

There are 12 months in a year, 12 hours on a clock, 12 zodiac signs, 12 eggs in a dozen, 12 days of Christmas, etc.

Thirteen, the number that follows, is considered unusual or lopsided.

The 13th guests at two ancient events wreaked havoc.

In the Bible, the 13th guest at the Last Supper is Judas, the person who betrayed Jesus.

And Norse legend says darkness followed after Loki, a mischievous god, crashed a dinner party and tricked a blind god into killing Balder the Beautiful, the god of joy and gladness.

Christians believe that Jesus was crucified on a Friday.

Friday the 13th is also thought to be the day when Cain killed his brother Abel.

Why are we so scared of Friday the 13th?

The superstition that 13 and Friday in combination is unlucky extends to Hollywood -- there's an entire horror franchise called "Friday the 13th."Eric Charbonneau | Invision via AP

First: Someone taught you about it.

"The fear of Friday the 13th is a cultural phenomenon that is widespread," says Stuart Vyse, psychologist and author of "Believing in Magic: The Psychology of Superstition." (Sorry: This report actually perpetuates the superstition.)

There's even an entire horror movie franchise called "Friday the 13th."

"If you grow up in a culture where superstition is talked about as real, then you're more likely to acquire that belief and be affected by it in your everyday life."

Second: It's a fearful superstition as opposed to a positive one.

Here's Vyse again:

"When you're taught a negative superstition like Friday the 13th, it brings its own anxieties with it. Once you're aware of it, once it comes to mind, if you've acquired that belief at all or you've been exposed to it, then suddenly you become anxious simply because you're now aware of it and have to deal with it.

"Even though rationally, (a person) may say 'this is silly,' there's an intuitive side and an emotional side of our reasoning that often says, 'You know what? I'm not going to take a chance. I'm going to avoid that doctor's appointment today or whatever in order to not take a chance on something happening.' "

Third: It's rare.

"If everyday was Friday the 13th, no one would care," says Vyse. "Therefore, it's notable when it comes up. ... Any fear that's associated with it is a bit more salient."

The missing floor

Some buildings -- like this one in downtown St. Paul -- and hotels don't have a 13th floor. They skip over it, going from 12 to 14.Nancy Yang | MPR News

The fear of 13 is enough to make entire floors disappear. Well, kind of.

Some buildings and hotels don't have a 13th floor, going from 12 to 14.

A 2007 USA Today/Gallup poll found 13 percent (coincidence?) of those surveyed would be bothered by staying on a room on the 13th floor.

Around the world

Friday the 13th? Pfft — at least in Spain. That's because their unlucky day is Tuesday the 13th.

It may have to do with the origins of the Spanish word for Tuesday, which is "martes," named after the Roman god of war Mars.

"In Spanish we have a saying that goes: 'En martes ni te cases ni te embarques, ni de tu casa te apartes," which means: "On Tuesday, do not get married or embark (on a journey, on a new enterprise, etc.) nor go too far from your house,' " said Ofelia Ferrán, an associate professor in the Department of Spanish and Portuguese Studies at the University of Minnesota.

"This reflects the feeling that it is not a good idea to start things on a Tuesday, or leave the space of safety that is the home, all because of the association of martes with the possibility of violence, unrest, war or general danger associated with the Roman god Mars."

Meanwhile, for Italians, the unluckiest day is said to be Friday the 17th.